A demand for high-density recording has been increased in the field of the magnetic recording medium. In order to meet the demand, it is necessary that the particle size of the ferromagnetic fine powder is made much finer, the dispersibility of the ferromagnetic fine powder is improved, the surface properties of the magnetic layer are improved and the filling degree of the ferromagnetic fine powder is improved.
JP-B-58-41565 (the term "JP-B" as used herein means an examined Japanese patent publication"), JP-A-57-44227 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-30235, JP-A-60-238306, JP-A-60-238309 and JP-A-60-238371 disclose the use of binders having a polar group such as --SO.sub.3 M, --OSO.sub.3 M, --PO.sub.3 M.sub.2, --OPO.sub.3 M.sub.2, --COOM or the like or binders having the polar group and an epoxy ring to improve the dispersibility of the ferromagnetic fine powder.
These binders have high adsorptivity to the ferromagnetic fine powder and good dispersibility can be imparted in comparison with conventional binders having only a polar group. However, even when these good binders are used, the finer the particle size of the ferromagnetic powder for the purpose of high-density recording the more difficult its dispersion. Accordingly, it is highly demanded to develop a method which allows the performance of highly dispersible binders to be exhibited.
JP-A-60-147931, JP-A-60-187931 and JP-A-62-143230 disclose methods wherein the addition stage of lubricants such as fatty acids are carried out after the dispersion stage of the ferromagnetic fine powder in the binders. However, magnetic recording mediums having high electromagnetic properties and improved dispersibility of ferromagnetic fine powder having a crystallite size of not larger than 500 .ANG. in particular can not be obtained by these methods. The effect on the fineness of the ferromagnetic powder for high-density recording in particular is not exhibited.
In the methods described in the above patent specifications, durability and running properties are insufficient and further deteriorated particularly when the surface of the magnetic layer is smoothed to improve electromagnetic properties.
JP-A-62-143230 proposes a process for producing a magnetic recording medium comprising a step of dispersing ferromagnetic powder in a solution containing a binder having sulfo group or phospho group in the form of an alkali metal salt, a step of adding a fatty acid to the resulting dispersion and coating the resulting magnetic coating solution.
Since sulfo group and/or phospho group are/is in the form of an alkali metal salt, the dispersibility of ferromagnetic powder is improved and the surface smoothness of the magnetic layer is improved. Further, since the fatty acid is added to the dispersion of the ferromagnetic powder, the fatty acid is not adsorbed by the ferromagnetic powder and stable running properties can be obtained.
However, when ferromagnetic fine powder having a crystallite size of not larger than 500 .ANG. is used, it has been found the S/N can not be satisfactorily improved, .mu. value is high and still durability is not sufficient.